Effects of pre-treatment and plasma enhancement on chemical vapor deposition of carbon nanotubes from ultra-thin catalyst films

We report a detailed study of surface-bound chemical vapor deposition of carbon nanotubes and nanofibers from evaporated transition metal catalysts exposed to ammonia diluted acetylene. We show that a reduction of the Fe/Co catalyst film thickness below 3 nm results into a transition from large diameter (> 40 nm), bamboo-like nanofibers to small diameter (similar to 5 nm) multi-walled carbon nanotubes. The nanostructuring of ultrathin catalyst films critically depends on the gas atmosphere, with the resulting island distribution initiating the carbon nucleation. Compared to purely thermal chemical vapor deposition, we find that, for small diameter nanotube growth, DC plasma assistance is detrimental to graphitization and sample homogeneity and cannot prevent an early catalyst poisoning. (c) 2006 Elsevier B.V. All rights reserved.

High rate deposition of Ta-C:H using an electron cyclotron wave resonance plasma source

A compact electron cyclotron wave resonance (ECWR) source has been developed for the high rate deposition of hydrogenated tetrahedral amorphous carbon (ta-C:H). The ECWR provides growth rates of up to 900 angstrom/min and an independent control of the deposition rate and ion energy. The ta-C:H was deposited using acetylene as the source gas and was characterized in terms of its bonding, stress and friction coefficient. The results indicated that the ta-C:H produced using this source fulfills the necessary requirements for applications requiring enhanced tribological performance.

Amorphous carbon-silicon alloys prepared by a high plasma density source

The addition of silicon to hydrogenated amorphous carbon can have the advantageous effect of lowering the compressive stress, improving the thermal stability of its hydrogen and maintaining a low friction coefficient up to high humidity. Most experiments to date have been on a-C1-xSix:H alloys deposited by RF plasma enhanced chemical vapour deposition (PECVD). This method gives alloys with considerable hydrogen content and only moderate hardness. Here, we use a high plasma density source, the electron cyclotron wave resonance (ECWR) source, to prepare films with a high deposition rate. The composition and bonding in the alloys is determined by XPS, visible and UV Raman and FTIR spectroscopy. We find that it is possible to produce hard, low stress, low friction, almost humidity insensitive a-C1-xSix:H alloys with a good optical transparency and a band gap over 2 eV.

Deposition of carbon nitride films using an electron cyclotron wave resonance plasma source

Hydrogenated amorphous carbon nitride (a-C:N:H) has been synthesized using a high plasma density electron cyclotron wave resonance (ECWR) technique using N2 and C2H2 as source gases, at different ratios and a fixed ion energy (80 eV). The composition, structure and bonding state of the films were investigated and related to their optical and electrical properties. The nitrogen content in the film rises rapidly until the N2/C2H2 gas ratio reaches 2 and then increases more gradually, while the deposition rate decreases steeply, placing an upper limit for the nitrogen incorporation at 30 at%. For nitrogen contents above 20 at%, the band gap and sp3-bonded carbon fraction decrease from 1.7 to 1.1 eV and approximately 65 to 40%, respectively. Films with higher nitrogen content are less dense than the original hydrogenated tetrahedral amorphous carbon (ta-C:H) film but, because they have a relatively high band gap (1.1 eV), high resistivity (109 Ω cm) and moderate sp3-bonded carbon fraction (40%), they should be classed as polymeric in nature.

Plasma enhanced chemical vapour deposited carbon nanotubes for field emission applications

Plasma Enhanced Chemical Vapour Deposition is an extremely versatile technique for directly growing multiwalled carbon nanotubes onto various substrates. We will demonstrate the deposition of vertically aligned nanotube arrays, sparsely or densely populated nanotube forests, and precisely patterned arrays of nanotubes. The high-aspect ratio nanotubes (∼50 nm in diameter and 5 microns long) produced are metallic in nature and direct contact electrical measurements reveal that each nanotube has a current carrying capacity of 107-108 A/cm2, making them excellent candidates as field emission sources. We examined the field emission characteristics of dense nanotube forests as well as sparse nanotube forests and found that the sparse forests had significantly lower turn-on fields and higher emission currents. This is due to a reduction in the field enhancement of the nanotubes due to electric field shielding from adjacent nanotubes in the dense nanotube arrays. We thus fabricated a uniform array of single nanotubes to attempt to overcome these issues and will present the field emission characteristics of this.

Electrical and field emission investigation of individual carbon nanotubes from plasma enhanced chemical vapour deposition

Plasma enhanced chemical vapour deposition (PECVD) is a controlled technique for the production of vertically aligned multiwall carbon nanotubes for field emission applications. In this paper, we investigate the electrical properties of individual carbon nanotubes which is important for designing field emission devices. PECVD nanotubes exhibit a room temperature resistance of 1-10 kΩ/μm length (resistivity 10-6 to 10-5 Ω m) and have a maximum current carrying capability of 0.2-2 mA (current density 107-108 A/cm2). The field emission characteristics show that the field enhancement of the structures is strongly related to the geometry (height/radius) of the structures and maximum emission currents of ∼ 10 μA were obtained. The failure of nanotubes under field emission is also discussed. © 2002 Elsevier Science B.V. All rights reserved.